The constantly increasing demand for high data rates in cellular networks requires new approaches to meet this demand. A challenging question for operators is how to evolve their existing cellular networks so as to meet the requirement for higher data rates. In this respect, a number of approaches are possible, namely: (i) increase the density of existing macro base stations, (ii) increase the cooperation between macro base stations, or (iii) deploy smaller base stations in areas where high data rates are needed within a macro base station grid. The last approach is referred to as a “heterogeneous network,” or “heterogeneous deployment,” where the heterogeneous network/deployment includes a macro cell layer (i.e., a layer of macro base stations serving corresponding macro cells) and one or more small cell layers (i.e., one or more layers of smaller, or lower power, base stations serving corresponding small cells). The small cells may sometimes be referred to as, for example, micro cells or pico cells.
The notion of a shared cell (also referred to as a “same cell,” a “merged cell,” or a “soft cell”) is one possible instantiation of a heterogeneous network. In a shared cell, a number of Receive/Transmit (R/T) points share the same cell Identifier (ID) as well as cell specific signals such that, from a wireless device (e.g., User Equipment device (UE)) perspective, these smaller “cells” served by the R/T points are seen as one effective cell (i.e., the same cell).
In a shared cell, several R/T points, each with their own coverage area, collectively serve a larger coverage area that is identified with a cell ID. Typically, identical signals are transmitted at each R/T point, though this is not required if there is sufficient Radio Frequency (RF) isolation between regions within the shared cell and/or if the information is scheduled over the air so as to avoid a wireless device receiving conflicting, non-resolvable information. The shared cell approach avoids the proliferation of cell IDs. Shared cells also avoid the high signaling load that would occur if each R/T point was a stand-alone cell and required hand-off operations as wireless devices moved through the general coverage area.
A wireless device connected to a shared cell does not distinguish between the different R/T points in the shared cell. However, the central processing node for the shared cell (e.g., an enhanced or evolved Node B (eNB) in 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE)) may or may not distinguish between the R/T points for uplink signals depending on whether separate lines are provided between the central processing node and the different R/T points. In this disclosure, two shared cell configurations are considered, and the following nomenclature is used:                Merged cell: A merged cell is a type of shared cell formed by multiple R/T points each sharing a common cell ID, transmitting a common signal set and, in the uplink direction, providing a common return for processing. In a merged cell, the central processing node is not able to distinguish between the different R/T points for uplink signals due to the fact that the uplink signals from the different R/T points are combined prior to processing.        Combined cell: A combined cell is a type of shared cell formed by multiple R/T points (or their equivalent) each sharing a common cell ID, but capable of transmitting a unique signal set and, in the uplink direction, providing a unique return for processing for each R/T point (or its equivalent).        
Currently, shared cell deployments are of particular interest for indoor systems. Indoor system deployments are becoming popular solutions for addressing the exponential increase of mobile data throughputs and the overload that macro systems are experiencing. Since the majority of mobile traffic is indoor, deploying indoor systems in buildings holding a significant number of mobile users will significantly increase the user experience for the indoor users and, at the same time, will off-load the macro network.
In a typical indoor deployment, the R/T points are separated by a distance on the order of approximately 10 to 30 meters (m); however, actual distances between the R/T points will be dependent on the specifics of the RF behavior and room layout in the particular indoor environment (e.g., building). Therefore, in a typical indoor environment, a wireless device will be relatively close to several R/T points in the shared cell and will provide good uplink quality signals to the receive antennas of many of the R/T points.
There are many potential applications for the use of the location of wireless devices in a cellular network. Examples include:                Enhanced 911 (E911) location for emergency services. In this regard, wireless device location performance capabilities are mandated by the Federal Communications Commission (FCC).        UE location for commercial and user applications that rely on UE position knowledge.        
There are several existing technologies, e.g., Global Positioning System (GPS), that can be employed to determine the location of a wireless device. However, in many cases, wireless devices will not have access to the technology. In particular, in an indoor environment, wireless devices do not normally have access to GPS technology. Multilateration is also a known technology for determining the location of a wireless device in a conventional cellular network (i.e., in a non-shared cell deployment). The basic concept of multilateration is the determination of the range between the wireless device of interest and a set of geographically distributed reference points having known locations. In LTE, downlink based multilateration techniques may be used (i.e., Observed Time Difference of Arrival (OTDOA)) by comparing Positioning Reference Signal (PRS) symbols between cells. Uplink LTE signals can also be used for ranging between cells (Uplink Time Difference of Arrival (UTDOA)), but this is more challenging since, unlike the downlink PRS reference symbols, the uplink signals are not designed to be normally hearable between cells. While at a high level, the processing involved in OTDOA and UTDOA is the same (ranging and multilateration), there are key differences in where the processing is performed and with the signals involved in processing.
While GPS and multilateration provide for wireless device location determination in a conventional cellular network, shared cell deployments present new issues. For example, as discussed above, in an indoor shared cell deployment, GPS is normally not available. Further, particularly for merged cells where the uplink signals from the different R/T points are combined before processing, conventional uplink multilateration cannot be used. As such, there is a need for systems and methods for determining the location of a wireless device in a shared cell deployment and, in particular, in a merged cell deployment.